The invention relates to a method and an apparatus for estimating the cardiac output of the heart of a patient, wherein an indicator is injected as an impulse in the blood stream of the patient, and a first signal corresponding with the concentration variation of the indicator is measured at a position at a distance downstream of the injection position, said first signal being supplied to a processor unit for determining the cardiac output from said first signal and the injected amount of indicator.
According to a known method of this kind, which normally is referred to as dilution method, a catheter is placed in the blood stream of the patient by means of which the indicator, for example a relatively cold fluid, salt, a colorant or the like, is injected in the blood, wherein said catheter comprises a detector downstream of the injection position for measuring the concentration of the injected indicator. In this manner a so-called dilution curve can be estimated which indicates the course of the indicator concentration. As the injected amount of indicator is known, the cardiac output can be determined from this information. In this case it is a precondition that the cardiac output is constant. In practice it is proved that the cardiac output pulsating due to the heart action can be considered as constant. However, when the patient is ventilated by a ventilation apparatus, the cardiac output is affected in such a manner that it can not be considered as a constant, but on the contrary shows a fluctuating character, whereas under these circumstances an accurate determination of the cardiac output is desired because the cardiac output is one of the criteria for determining the condition of the patient. Experiments have shown that at the determination of the cardiac output by means of this known method, the measuring results can show a spread of 65-125% of the average value.
According to an other known method, normally referred to as pulse contour method, a catheter is placed in the blood stream of the patient, by means of which a signal is measured, which signal is substantially proportional to the cardiac output. For the estimation of the cardiac output it is assumed that the characteristic impedance of the vascular system downstream of the measuring place is constant. However, this condition is not fulfilled in practice because said characteristic impedance is varying with a relatively large time constant. Thereby, reliable measuring results cannot be obtained by this known method.